Your Sister’s Sister, which opens today, was one of the stronger films SPIN saw at Sundance this year, though you might not trust this based on the description alone: After the loss of his brother, Jack (mumblecore mainstay Mark Duplass) retreats to a secluded cabin belonging to his friend Iris (Emily Blunt) where, unbeknownst to him, Iris’s half-sister (Mad Men‘s Rosemarie DeWitt) is already staying. That’s all that can really be said without spoiling what little there is to spoil, but writer-director Lynn Shelton (Humpday) has a knack for eliciting the most disarming performances from her three leads that these characters are immediately familiar. (Blunt, seemingly modest and elusive by nature, is especially charming.) As with all of Shelton’s films, the dialogue is mostly unscripted, which led to some exhausting navel-gazing in 2008’s My Effortless Brilliance, but here, like in 2009’s surprisingly good Humpday, it adds a trustworthiness to this small story of siblings and loneliness and love. SPIN talked to Shelton about the ambiguous ending and the dreaded M-word.

The story felt very personal. Was it? No, I came at the movie from an outsider’s point of view. I do have siblings myself, but they’re all very uncomplicated and boring. But I’ve observed some really dark, complicated sibling relationships over the years around me and been very compelled by them. And I invited the actors into the development process to talk about back story and what might have happened between these people and who these characters are.

So how did you come to this particular idea? The starting point is Mark. He called me and said that he had this vault of movie ideas with his brother [collaborator/director] Jay. This movie in particular didn’t seem like anything they’d be making any time soon, because it was about a guy who just lost his brother, and it felt a little too close to home to them. When he brought it to me, it didn’t involve sisters at all. Instead of the older sister being at the cabin, it’s the girl’s mother. The very first thing I did was change the mom to a sister. It attracted me more. And I liked the idea of there being parallel relationships, because to me, the fourth character in the film is the dead brother. He’s the reason Jack and Iris are friends to begin with, and he’s why Jack doesn’t want to get in between the two sisters.

What percentage of the script was written? Somebody will have to send a mathematician in to figure it out, but we’ve been estimating that 75 to 80 percent of it is not scripted. I had pages of dialogue written, and that was mostly for the actresses, who were not veteran improvisers. They were already terrified. I didn’t want them to feel I was throwing them out without any kind of safety net. But I said, “Please don’t memorize them.”

Is there a technique to that? Oh for sure, it’s a very conscientious formula. You work really hard as an actor on my set. The normal ratio of work for an actor on a film set is two hours out of a 12 hour day because there’s so much work that goes into lighting and getting that crane shot set up. And on my set you’re spending a good 10 or 11 hours acting your butt off. I very carefully select everybody who’s going to be there, people who are going to have a good chemistry with everybody else, a good sense of humor. It feels like film camp. I want a very small, intimate group of really awesome people who are sensitive to the fact that the whole movie is focused on the actors and on the performances.

Is the cast that you have the one you pursued? No, the first call I made after Mark was Rachel Weisz. We’d been wanting to do something for a long time. So the older sister was going to be played by Rachel. Three days before the shoot, we lost Rachel and I replaced her with Rose. It was traumatic but ultimately serendipitous, and it’s so impossible for me to imagine anyone else in that role now.

Are you against imagining how it ends? I mean I love that it’s a choose your own adventure ending, and in case it felt too brutal, we had a script prepared, and I tried it out on an audience, and they just hated it. So that was a very early decision.

The word “mumblecore” is sort of reviled among all the people who are associated with it. But is it helpful in terms of creating curiosity? It was very useful early on in all of our careers because we were making these little tiny movies that wouldn’t have gotten a ton of attention otherwise. So to be associated with this American independent movement, even if it was given the most unfortunate name in the freaking universe, is not a problem for me. I’ve never met [Andrew] Bujalski [(Mutual Appreciation, Funny Ha Ha)], but everybody else I know and love and respect. It’s a little bit reductive. But we all are in a quest for a certain kind of truth: We want to see recognizable human beings on screen.

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